A PLC controls an arithmetic device to execute a control program stored therein to generate transmit data, and transmits the transmit data as indivisible data to another PLC or the like via a transmission line. In this explanation, the indivisible data mean a series of data that is constituted by a plurality of pieces of information, the original meaning of which can not be grasped if the data are divided. Therefore, it is necessary to transmit the indivisible data without being divided.
Different settings are made for a time required to execute one cycle of a control program and a timing of generating transmit data during the execution of the control program among control systems each including the PLC, and the time and timing can vary from time to time during the execution of the control program. Meanwhile, transmit data generated by the arithmetic device are transmitted to a transmission line according to a transmission system (such as Time Division Multiple Access: TDMA) specified for the transmission line in advance. Therefore, it is often required for the arithmetic device to asynchronously carry out generation of the transmit data and transmission of the transmit data to the transmission line. Even in this case, it is necessary to guarantee to transmit the transmit data as indivisible data (hereinafter, such transmission condition is referred to as “simultaneity of the indivisible data”) when transmitting the transmit data to the transmission line.
As a technique for solving such a problem, there is a technique disclosed in Patent Literature 1.